1. Field of the Invention
This invention relates generally to an integrated optical polarizer for high index contrast SOI waveguides, and more particularly to an integrated optical polarizer that operates through coupling of TM-polarized guided mode to a gap plasmon-polariton mode.
2. Description of the Related Art
Silicon-On-Insulator (SOI) platform is expected to become a common basis for integrated optical systems used in fiber optic communication industry. Its main advantage is compatibility with well-established complimentary metal-oxide-semiconductor (CMOS) fabrication technologies which have been proven to facilitate high volume manufacturing of highly integrated electronic devices and systems. SOI waveguides possess unusually high refractive index contrast (n(Si)≈3.48, n(SiO2)≈1.44). This provides a very strong confinement for guided modes and eventually leads to a very high degree of integration, which is hardly achievable using traditional integrated optical material systems. SOI-based integrated optics quickly evolved into a separate field—silicon photonics. At the telecom wavelengths, many silicon photonic devices have been successfully demonstrated including optical multiplexers/demultiplexers based on arrayed waveguide gratings, silicon chip Raman lasers, photonic circuits based on photonic crystals, electro-optical modulators, narrowband filters, and others.
Polarization control is an essential component of integrated optical systems. Several schemes have been proposed for integrated optical polarizers implemented in SOI. The lateral Bragg reflector structure made of Si/Si3N4 has been shown to be polarization-sensitive, especially when light approaches Si/Si3N4 interfaces at the Brewster angle. In such a structure, also known as antiresonant reflecting optical waveguide, light is confined to a low-index core. The lateral confinement requires several Bragg periods and thus can never be really strong. The vertical antiresonant reflecting optical waveguide structure is also polarization-sensitive, and it also provides weak light confinement to a low-index core. The scheme employing 3-dB multi-mode interference couplers and Mach-Zehnder interferometer with rib waveguides of different width relies on birefringence in the rib waveguides. It works in a spectral range of about 45 nm. The Mach-Zehnder interferometer with multi-mode interference couplers altogether has a relatively large footprint. The scheme that uses vertically coupled microring resonator is expected to provide 20 dB polarization splitting ratio, however, in a very narrow range of wavelengths.
The above schemes are a lot more complicated compared to a simple and elegant integrated optical polarizer in which transverse magnetic (TM) polarized light is coupled to a surface plasmon-polariton mode, which then quickly decays due to losses in the metal. Transverse electric (TE) mode in a system made of isotropic materials cannot be coupled to the plasmon-polariton and thus propagates with much smaller losses. These polarizers are known since 1970s. It worth mentioning that some polarization discrimination is known to exist in waveguides with metallic claddings even if there is no coupling to plasmon-polaritons: due to different penetration of light into the metallic cladding, losses for TE-polarized mode are lower compared to losses for TM-polarized mode. Thus, the simplest structure of an integrated optical polarizer would contain just a metallic cladding over dielectric guiding layer. By introducing appropriately designed dielectric cladding layer between the guiding layer and the metal, efficient interaction is facilitated between the mode confined to the guiding layer and the plasmon-polariton at the metal/cladding layer interface, leading to dramatic improvement of the polarizer performance.
Unfortunately, such approach cannot directly be applied to the SOI waveguides because it is difficult to match the propagation constants of a guided mode in an SOI waveguide and a surface plasmon-polariton mode. The problem is that the modal index of the guided mode is large (comparable to the refractive index of silicon), while modal index of the plasmon-polariton is low, being just slightly above the refractive index of the dielectric cladding layer between the silicon guiding core and the metal layer. On the other hand, metal-dielectric nanoscale multilayers have been shown to support high-index plasmonic modes. In a structure made of three pairs of silica(˜29 nm)/gold(˜25 nm) nano-layers, guided modes with indices 2.31 and 2.88 have been experimentally verified. The simplest high-index plasmonic mode is a gap plasmon-polariton supported by a thin dielectric gap layer between metallic layers.
It is, therefore, an object of this invention to provide a polarization arrangement that requires but a small footprint.
It is another object of this invention to provide a simple integrated polarization arrangement.
It is also an object of this invention to provide a polarizer arrangement that can be operated over a relatively broad ranged of frequencies
It is a further object of this invention to provide a polarizer arrangement that the overall design can be completed by simple adjustment of the thicknesses of the layers.